The biggest peak on an fft indicates maximum energy. The peak around 100Hz is the main resonant frequency.

-3dB is roughly 1/2 the energy of 0dB. And each incremental 3dB is another 1/2 of that (-6dB is 1/4 intensity). So the peak where your cursor is located at -32dB is around 1/1500 of the intensity of the peak that is 0dB.

In this case, it's not really useful to look for the 'main' resonant frequency, and this sort of plot is not very helpful in any case without a lot more information.

'Free' backs, and tops as well, have a lot of different resonant modes. Which one predominates in the response depends on how the plate is held, where it is tapped, and where the microphone is. Even a small change in any of those parameters can result in a totally different set of peaks in the FFT. All of the spetra are 'valid' and give you some information, but it's far from obvious how you sort them out to extract any meaning that can guide you.

A better way to sort resonances, IMO, is by Chladni patterns. You not only get fairly exact frequency information, but the mode shapes as well. My experience suggests that 'free' plate mode shapes tell you more about how the guitar will end up than the pitches.

OTOH, I've been using the Chladni method for a long time, and am still far from convinced that I know just what to do about the back. On one fairly recent guitar that turned out well the back modes that seemed most indicative were the 6th and 8th modes, and were up around 250-350 Hz. The 'fundamental' mode, was down at 44 Hz, and was a twisting mode that tells very little about how the plate will vibrate when the guitar is together. The next one up, at 50 Hz, is a bending mode with more potential utility, but not much more. Either of them could produce a strong peak in a hold-and-tap test, depending on how you do it.

Is the back braced yet or without braces? Once I brace my backs I look for two modes that I use to tune the back--like Alan said there are a bunch but I look for the peak around 40-50hz, which is the fundamental mode and seems to be affected the most by along the grain stiffness or how thick I leave the back. I also look for the mode around 270hz--the one Alan mentions. I find that mode is influenced strongly by how stiff the back cross braces are, and I'll shave down the braces until I'm in the 270-285hz range give or take. In my design if the first hz is around 45 and the second around 280, my main back mode or T1,3 once assembled will be around 260hz or so. That's influenced by a lot of factors but that's what happens for me.